5-Hour Energy has annual sales of more than $1 billion and sells about 9 million bottles of the energy shot a week in North America. / Spencer Platt, Getty Images

by Elizabeth Weise and Jayne O'Donnell, USA TODAY

by Elizabeth Weise and Jayne O'Donnell, USA TODAY

The Food and Drug Administration said Thursday that it is investigating 13 reports of deaths and 33 hospitalizations linked to 5-hour Energy, the highly caffeinated shots that are often sold in convenience stores.

The news comes a month after Monster Energy drinks were linked to five deaths in FDA's so-called adverse-event reports.

Energy shots come with a "higher risk of health problems," says Amelia Arria,
a University of Maryland public health epidemiologist who has written several
energy-drink studies.

"The caffeine concentration is a lot higher because the volume of the liquid
is lower," says Arria. "Concentration is the issue, really."

The FDA does not regulate caffeine in energy drinks, which can be marketed
as dietary supplements. Sen. Richard Blumenthal, D-Conn., says he is concerned
about natural ingredients that also act as stimulants in the drinks. Experts
say this combination can be risky for people with undiagnosed heart conditions.

Soft drinks, on the other hand, are regulated and can have no more than 71.5
milligrams of caffeine in a 12-ounce serving.

Because they don't know how much caffeine is in energy drinks, "consumers may
be at risk for caffeine toxicity," says Laura Juliano, an American University
associate professor of psychology who is an expert on cognitive and behavioral
mechanisms that influence caffeine.

Symptoms of caffeine toxicity, which is also called caffeine intoxication,
range from nervousness to cardiac arrhythmia. There have also been reports of
seizures, psychotic symptoms and death after drinking energy drinks, Juliano says.

Even when energy-drink companies do list the amount of caffeine, testing by
Consumer Reports magazine found that many drinks contain more caffeine
than is listed on their labels, says health editor Gayle Williams. Five of the
drinks they tested had "an amount of caffeine more than 20% above what was on
the label," she says.

"We tell people 'read the label,' but if the labels are inaccurate, that's really
not helping you," Williams says. "It's very difficult to recommend these because
people don't necessarily know what they're getting. And in some cases not knowing
what you're getting can be potentially fatal."

Consumer Reports says 5-hour Energy has 212 milligrams of caffeine,
though the drink does not list the amount of caffeine it contains. For comparison,
an 8-ounce cup of coffee typically contains about 100 milligrams of caffeine.
A 16-ounce Starbucks Grande contains 330 milligrams.

Monster Beverage doesn't list the amount of caffeine in its products. The company
told Consumer Reports that's because there are no laws or business regulations
requiring them to and that the numbers wouldn't matter to most consumers anyway
because their products are safe.

The existence of an adverse-event report "does not necessarily mean that the
product identified in the report actually caused the adverse event," the FDA
said in a statement. "If we find a relationship between consumption of the
product and harm, the FDA will take appropriate action to reduce or eliminate
the risk."

FDA cautioned consumers they should consult with their doctors before drinking
energy drinks "to ensure that there are no underlying or undiagnosed medical
conditions that could worsen as a result of using them."

"If somebody isn't used to having caffeine and if they take a big dose of it,
if they have an pre-existing cardiac problem, which they might not have been
aware of, it could lead to a life-threatening abnormal heart rhythm, which
can lead to sudden death," says Byron Lee, a professor of medicine in the division
of cardiology at the University of California-San Francisco.

Arria says regulators need to "re-examine whether it's appropriate" to have energy
drinks in their own regulatory category given their widespread usage by consumers,
including young people. "In general, there's a need to look at the way that
we regulate products that are not strictly foods or beverages," says Arria.

Juliano agrees. "When manufacturers add caffeine to products, I believe that
they should be required to indicate the amount of caffeine on product labels,"
she says. "This way consumers can make an informed choice and be aware of their
caffeine exposure."

The New York Times was first to report on the 5-hour Energy death investigations.